Worker's skills can be divided into two categories - skills that are general, and could be used by any employer in the same field; And skills that are specific, and apply ONLY to their current employer. It is these latter skills that are the reason why a guy with a few years of experience on the job is more effective than a new hire, no matter how qualified or experienced that new guy might be.
I was with my previous employer for over ten years, in a variety of positions as I moved up through the company, and by the time I left, I had an encyclopaedic knowledge of the fine details of the way the company worked - from a knowledge of who worked in what departments, and who would actually be willing and able to answer a given question, who would be able but unwilling, and who would be willing but unable; Through to an ability to instantly know which generic product names mapped to which of our (several hundred) finished product codes, product tradenames, and/or major competitor brandnames. All of that knowledge was of great value to that company, but of no value to any other potential employer - the few direct competitors who might have found some of it useful I was forbidden from working for by non-compete clauses in my contract.
Employers are in a monopsony position as regards this kind of institutional knowledge (dismal's ignorance-based faux-incredulity notwithstanding), and many employees have few documented skills that they can effectively use to sell themselves to a different employer. When only your current employer knows your true worth, and only your current employer is able to leverage a sizable fraction of your accumulated experience and skills, moving between employers is bad for everyone involved. The employer you leave loses your valuable institutional knowledge, and must pay your replacement to do a less effective job while he acquires this; You lose the value of those employer specific skills, and have to start again at the bottom; And overall productivity drops as a result of both factors.
General skills that can be used by any employer (or any employer in a specific field) are less impacted by this, but even there, incumbency counts - if you have been working with a particular company for a while, they know not only that you have a degree in applied whatever-they-do-for-a-business-ology; But also how competently you perform tasks related to that supposed knowledge. Two applicants with identical paper qualifications might have widely differing skill levels - some people get a degree as a result of their fascination with the subject matter, and others get a degree so that they can find work, and forget as much of their learning as possible the day after the final exam. Those most valuable employees who fall into the former group can never effectively demonstrate that level of value to anyone other than their current employer, and so are at a disadvantage in the job market, in that they have greater value, but appear to be equal to everyone else. They can only leverage their actual value by remaining with their current employer. If that's not another form of monopsony - having a product that you can only sell to one buyer - then what is it?